The present invention relates to a method for producing nanostructures starting from compounds that are in a hexagonal crystalline shape.
It relates in particular to a method for producing nanobeams, nanofilaments, nanotubes, bundles of nanotubes, twists, nanoribbons, onions, hollow spheres, clusters, compounds of boron nitride either pure or loaded with certain elements.
The invention also relates to the nanostructures obtained by said method.
Structures and nanostructures in the form of fullerenes, hollow nanospheres, or nanotubes of carbon or assemblies of these nanotubes called cords or bundles, are known. These carbon nanostructures are in fact anisotropic forms produced starting from graphite. It will be recalled that carbon nanotubes are in fact one or more graphene sheets rolled up together, with a more or less marked degree of helicity (which can also be zero).
Numerous methods of production have been developed for obtaining carbon nanostructures, such as, in particular the use of an electric arc, the catalytic decomposition of gases, for example hydrocarbons, laser ablation, or laser bombardment of a gas. The method most widely used, on account of its efficiency and the intrinsic properties of graphite (especially its good conductivity) is the electric arc method. As a rule it has to be followed by processes of purification or filtration to obtain nanotubes of a single type.
Since boron nitride has a crystalline structure similar to that of graphite, there have been attempts to obtain nanostructures from this compound. Obviously, the physical and chemical properties of boron nitride are totally different from those of carbon; it should be noted in particular that the melting point of boron nitride is about 1000xc2x0 C. lower than that of carbon. In addition, it is much more insulating from the electrical standpoint and chemically it is much more ionic than carbon, and less reactive with a certain number of elements. These differences with respect to physicochemical properties between these two elements make the techniques known from the prior art, such as the use of hafnium diboride as the electrode in the electric arc method for example, rather inefficient.
Accordingly, the present invention aims to overcome the drawbacks of the known methods of the prior art, in that it proposes a method for producing nanostructures in large quantity and having, if desired, a high degree of purity.
The method according to the invention for producing nanostructures under a controlled atmosphere, starting from compounds having a hexagonal crystalline shape, subjected to gas laser bombardment, is characterized in that a compacted sample of compound is employed and the operations are carried out at a residual gas pressure between about 5.103 and 8.104 Pa. The vacuum can be dynamic or not.